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Crustal reactions resulting from the mid-Pliocene to Recent continent-island arc collision in the Timor region

Abstract

The Sunda and Banda Arcs are contiguous surface features associated with the convergent boundary between the Southeast Asian
and Australian-Indian Ocean plates, and where these two arcs meet, the tectonic environment changes from oceanic subduction
to continent-island arc collision. This area has been investigated in an attempt to recognise and understand some of the effects
of the introduction of continental crust into a subduction zone. Depositional environments encountered in Deep Sea Drilling
Project (DSDP) hole 262 have been correlated with present environments to determine past horizontal distances between the
leading edge of the subduction zone and the DSDP location on the Australian crustal margin. These data have been combined
with the apparent plate motion at this margin to derive an estimate of the surface width of the subduction zone through time.
The similarity between past width variations and present lateral variations has been used as the basis for proposing a collision
model. Before the collision, the Indonesian subduction zone extended eastwards into the region that is now the southern Banda
Arc. The continental edge of Australia first entered this subduction zone about 3 m.y. ago, in the mid-Pliocene. Initially,
a wedge of deformed continental margin sediments began to develop at the leading edge of the subduction zone. The tectonic
front that separated deformed from undeformed sediments and an associated bathymetric low migrated up the continental slope
leaving in their wake a large wedge of deformed continental margin sediments, which produced a southerly bulge in the subduction
zone. Ultimately, the deformation wedge began to absorb the near-surface stresses, and relative motion between the front and
the southern plate slowed to near zero. Continuing relative plate motion carried the deformation front back towards the volcanic
arc. Compression from this has been almost entirely taken up in the subduction zone, and has led to thickening and uplift
of the deformation wedge and crust associated with the pre-collision outer-arc ridge. The proposed collision model explains
many of the morphological, geological, and geophysical irregularities in the Timor region. On a broader regional scale, the
evidence that the Southeast Asian plate at Timor is moving eastward at about 60 km / m.y. relative to the Eurasian plate,
supports the interpretation that the India/China continental collision is pushing the Southeast Asian plate to the southeast.